The field of wireless communication includes many wireless applications such as voice communication, paging, packet data services, and voice-over-IP. One challenge presented by such services are the widely varying requirements for capacity, quality-of-service, latency, data rates in the different services. Various over-the-air interfaces have been developed to accommodate combinations such services using different wireless communication techniques such as frequency division multiple access (FDMA), time division multiple access (TDMA), and code division multiple access (CDMA).
In order to accommodate combinations services having different sets of requirements, communication standards such as the proposed cdma2000 and W-CDMA specify the use of orthogonal codes of varying length on the downlink channels from a wireless base station to a subscriber station. Some standards also specify transmitting signals for different uplink channel (in the direction from the subscriber station to the base station) using orthogonal codes of varying lengths for the different channels. For example, a wireless base station may transmit three types of downlink signals, pilot, voice, and packet data, using a different length orthogonal code symbols to channelize or “cover” each different type of signal. The length of an orthogonal code symbol is typically described as a number of “chips,” with a chip being a smallest binary component of a transmitted signal. In a spread spectrum system, each bit of information is multiplied by a sequence of binary chips having a predetermined number of chips-per-bit. Multiplying a single binary information bit by an orthogonal code symbol effectively “spreads” the information bit over all of the chips in the symbol. For this reason, the chip length or chips-per-bit is often referred to as a “spreading factor” of a transmitted information signal.
Another aspect of current spread spectrum systems is the sharing of frequency bands between different subscriber stations and between different base stations. In other words, neighboring base stations in a spread spectrum system transmit their downlink signals in the same frequency band as each other. Because of this sharing (also called “reuse”) of frequency bands, downlink signals transmitted by a base station destructively interfere with the downlink signals of neighboring base stations. This interference decreases the capacity of the neighboring base stations, measured in either number of subscriber stations that can be supported or maximum information throughput possible on the downlink. The capacity of such systems can be increased by using power control techniques to decrease the transmit power of all signals to a lowest value that will still permit the signals to be correctly received and decoded by subscriber stations. The effectiveness of power control depends largely on the accuracy of noise measurements made by subscriber stations and base stations. There is therefore a need in the art for a way to provide maximal noise measurement accuracy in systems utilizing varying spreading factors.